The present invention relates to improving the solubility of dopants in silicon (Si), germanium (Ge), and silicon-germanium alloys (SixGel1-x) and particularly to enhancing solubility of dopants by subjecting the silicon based substrate to an appropriate strain, and more particularly to a method of enhancing solubility of dopants in silicon wherein the type of strain (compressive or tensile) is governed by the dopant""s charge and its size-mismatch with the Si-based substrate.
Ion implantation allows for exceptional control and reproducibility in the introduction of dopants into the near-surface region of semiconductors. As a result, it has been the universal method of choice for doping MOS transistors in silicon-based integrated circuits since the beginning of the semiconductor revolution. However, an undesirable effect of ion-implantation is that it introduces significant damage into the silicon wafer in the form of point defects and their dusters, Fahey et al. Rev. Mod. Phys. 61, 289 (1989). For a device to be operational these defects must be removed and the dopants electrically activated through high-temperature annealing. The annealing procedure leads to unwanted dopant diffusion, as well as nucleation and growth of dopant clusters and precipitates which results in incomplete activation.
Experience has shown that the solubility of boron in silicon under non-equilibrium thermodynamic conditions that prevail during the annealing procedure, i.e., in the presence of excess silicon self-interstitial atoms, is lower than its equilibrium thermodynamic value. The latter thus determines an upper bound for the concentration of substitutional B atoms in silicon. As technology continues to evolve toward smaller and faster transistors, this limit may soon be reached unless new ideas and/or technologies are brought forward that can reduce dopant diffusion during processing while at the same time increasing their electrical activity, see Packan, Science 285,2079 (1999).
The most widely used p-type dopant, i.e., boron, has a maximum solubility of less than 1 at. % in silicon at the annealing temperature of interest. This sets the limit for the highest concentration of electrically active boron impurities that can be reached with current implantation techniques. Already the next generation of transistors will be dangerously dose to this solubility limit. Another p-type dopant candidate with excellent diffusion properties, i.e., indium, has been used only on a small scale mainly because of its very low solubility in silicon. Thus, there is a need to remedy this acute problem faced by the semiconductor industry.
Recently the invention described and claimed in the above-referenced copending U.S. Application provides a solution to the above-referenced problem by a method for enhancing the solubility of boron and indium in silicon. That invention, like the present invention is based on the use of first-principles density-functional theory (DFT) in the local-density approximation (LDA) to calculate the temperature dependence of the equilibrium solubility of boron in crystalline silicon under various strain conditions. Verification of the above-referenced invention has shown that the equilibrium thermodynamic solubility of significantly size-mismatched dopants in silicon, such as boron or indium, can be raised by more than 100% if the silicon substrate is strained appropriately.
The present invention constitutes an improvement over the method described and claimed in the above-references copending Application. In this invention, it has been determined that the mixing properties of dopants in silicon (Si), germanium (Ge), and silicon-germanium alloys (SixGel1-x) are primarily governed by their charge and to a second order by their size-mismatch with the silicon-based substrate, as in the method of the above-reference application. Therefore, the solubility of small p-type (e.g., boron), as well as large n-type (e.g., arsenic), dopants can be raised most effectively by appropriate bi-axial strain, i.e. compression for the former, and tensile strain for the latter. Thus, while quantum mechanical calculations have shown an unexpectedly large increase on the order of 100%xe2x80x94in the solubility of boron in silicon upon only 1% compression (bi-axial) strain of the  less than 100 greater than  layers, this large enhancement has been found mostly due to the negative charge of the boron impurities in silicon, than boron""s previously considered size-mismatch with silicon.
It is an object of the present invention to increase the solubility of dopants in semiconductor substrate materials.
A further object of the invention is to provide a method for enhancing the solubility of dopants in silicon, germanium and silicon germanium alloys.
A further object of the invention is to provide a method whereby solubility of small size-mismatch p-type (e.g, boron) as well as large size-mismatch n-type (e.g., arsenic) dopants can be raised effectively by appropriate bi-axial strain.
Another object of the invention is to provide a method for enhancing the solubility of dopants in silicon, germanium and silicon germanium alloys utilizing both the charge of the dopant and its size-mismatch with the silicon based substrate to determine the appropriate strain to be applied to the silicon based substrate.
Another object of the invention is to increase solubility of boron and arsenic by appropriate bi-axial strain based on their charge and size-mismatch with silicon and germanium.
Another object of the invention is to provide a method for enhancing the solubility of dopants in silicon which takes into account the electronic structure of silicon due to which negatively charged impurities become more soluble under compressive strain while positively charged dopants prefer tensile strain.
Another object of the invention is to provide a method for enhancing the solubility of dopants in silicon utilizing strain based on the electronic structure of the dopant and the silicon, and the size-mismatch between the dopant and the silicon.
Other objects and advantages of the invention will become apparent from the description and accompanying drawings. The present invention is directed to enhancing the solubility of dopants in silicon, germanium and silicon germanium alloys. Prior quantum mechanical calculations, see above-referenced copending application, have shown an unexpectedly large increase on the order of 100%xe2x80x94in the solubility of boron in silicon upon only 1% compression of the  less than 100 greater than  silicon layers. This large enhancement was previously considered as being caused by the size-mismatch between boron and silicon atoms, boron being smaller. It has now been discovered that this large enhancement is, however, mostly due to the negative charge of the boron impurities in silicon. This charge arises as a result of the electronic structure of silicon due to which negatively charged impurities become more soluble under compression strain while positively charged dopants become more soluble under tensile strain. This finding can be extended to the case of dopants in Ge as well as silicon germanium alloys. Thus, it has been determined that the mixing properties of dopants in silicon, germanium and silicon-germanium alloys are primarily governed by their charge, and to second order by their size-mismatch with silicon. Thus, it has been established by this invention that the solubility of small p-type (e.g., boron) as well as large n-type (e.g., arsenic) dopants can be raised most effectively by appropriate bi-axial strain. It has also been determined that for large p-type dopants, such as indium, this effect is significantly reduced due to the competition from the charge and the size-mismatch which favor opposite strain types.